As tbworld wrote "Once you understand the fundamentals of programming languages", I did the same. For years I was developing PC Client-Server applications. Started with dBaseII but really grew-up with Clipper.
With PHP I have very short experiance mostly modifying the designed which at the early years (2000-2003) was including in the core files.
What makes me start learning PHP was a simple request that I post here (as paid mod). I wanted to add a very-very simple Dating section in my forum. Actually it was to add 5 fields in the user table, a Add/Edit form, and a 2 fields search. Doing it as PC application should not needed more than 3-4 hours.
I've been so surprised with the quote that I got. Still remember it even if it's exactly 10 years ago "We're a US based company bla...bla..bla... and our team (not just a coder), needs 2 months to complete your project which will costs you $2,000 !!!".
That was enought. I was sure that by knowing database programming should helps me, and I was right. I had my dating script ready in 10 days, with much more features. This was also my first mod that I post here (vbMates).